Two families of gene regulators play an important role in cellular signaling processes in vertebrates: the nuclear factor kappa B (NF-kappa B)/Rel group of transcription activators and their coevolved regulatory proteins, the inhibitors of kappa B (I kappa Bs). The biological functions of NF-kappa B comprise communication between cells, embryonal development, the response to stress, inflammation and viral infection, and the maintenance of cell type specific expression of genes. In several pathogenic conditions components of the NF-kappa B system are deregulated and could thus present potential diagnostic probes or targets for therapeutic intervention.